Shaft bearing arrangement

ABSTRACT

A gas turbine engine ( 10 ) comprising a first turbine ( 19 ) and a first compressor ( 13 ) mounted on a first shaft ( 26 ) and a bearing arrangement ( 52, 34 ) supporting the shaft ( 26 ), the bearing arrangement is subject to a total end load and comprises a fixed bearing ( 34 ), a load share bearing ( 52 ) and a bearing end load management system ( 50 ) that is capable of applying a variable load to the load share bearing to manage total end load between the bearings. The bearing end load management system comprises a static diaphragm ( 54 ) that defines part of a chamber ( 56 ) and that is coupled to the load share bearing whereby increased pressurization of the chamber loads the diaphragm which in turn increases the proportion of the total end load on the load share bearing.

The present invention relates to a bearing arrangement for a shaft andin particular, but not exclusively, to a passive bearing end loadmanagement system for bearings on which a shaft is supported in a gasturbine engine.

Conventional turbine engines comprise a shaft supported by a thrustbearing, which accommodate the net resultant axial force from say a lowpressure (LP) turbine and a compressor, and a roller bearing, which onlyradially locates the shaft. With the increasing desire for more powerfulengines reduction of core engine diameter and increased core enginerotational speed mean that the life of a thrust bearing is greatlyreduced. Servicing intervals become shorter and costs thereforeescalate. Furthermore, as a bearing's rotational speed increases as wellas its diameter increasing the centrifugal force of each rolling elementin the bearing becomes very significant in the bearing's life.

To overcome some of these problems a pressurised chamber is provided totransfer some of net LP turbine load to the engine mounts, therebyreducing the total load on the thrust bearing. However, and as will bedescribed in more detail in the description relating to FIG. 1, thepressurised chamber requires rotating seals which significantly reducethe engine's performance and efficiency.

Therefore it is an object of the present invention to provide a bearingarrangement for a shaft which obviates the above problems.

In accordance with the present invention there is provided a gas turbineengine comprising a first turbine and a first compressor mounted on afirst shaft and a bearing arrangement supporting the shaft about anaxis, the bearing arrangement is subject to a total end load andcomprises a fixed bearing, a load share bearing and a bearing end loadmanagement system that is capable of applying a variable load to theload share bearing to manage total end load between the bearings, thebearing arrangement is characterised by the bearing end load managementsystem comprising a static diaphragm that defines part of a chamber andthat is coupled to the load share bearing whereby increased/decreasedpressurisation of the chamber loads the diaphragm which in turnincreases/decreases the proportion of the total end load on the loadshare bearing.

Preferably, the gas turbine engine comprises a second turbine and asecond compressor mounted on a second shaft, the chamber is pressurisedby gas from the second compressor.

Preferably, the fixed bearing is mounted between the first shaft and thesecond shaft.

Preferably, the load share bearing is connects between the diaphragm andthe shaft.

Preferably, the chamber is further defined by a static structure; thestatic structure and the diaphragm are slidably sealed to one another toallow axial movement therebetween.

Preferably, two seals are defined between the diaphragm and the staticstructure.

Preferably, a pipe extends across the chamber to allow substantiallyequal pressure either side of the chamber.

A broader aspect of the present invention is a bearing arrangement of ashaft having an axis, the bearing arrangement is subject to a total endload and comprises a fixed bearing, a load share bearing and a bearingend load management system that is capable of applying a variable loadto the load share bearing to manage total end load between the bearings,the bearing arrangement is characterised by the bearing end loadmanagement system comprises a static diaphragm that defines part of achamber and is coupled to the load share bearing wherebyincreased/decreased pressurisation of the chamber loads the diaphragmwhich in turn increases/decreases the proportion of the total end loadon the load share bearing.

Preferably, the proportion of the total end load on the load sharebearing is greater than 50% and more preferably, between 55% and 85%.

Preferably, the diaphragm is annular and comprises at least one bend andmay comprise at least one flat portion. Preferably, the at least oneflat portion is positioned radially outwardly of the bend.

Preferably, a pipe comprises a valve to control the flow of pressurisedgas into the chamber.

Preferably, a number of pipes, each comprising a valve to control theflow of pressurised gas into the chamber, are connected to a number ofdifferently pressurised gas sources.

In another aspect of the present invention there is provided a method ofoperating a bearing arrangement as described in the above paragraphs,wherein the method comprises the step of pressuring the chamber above apredetermined limit.

The present invention will be more fully described by way of examplewith reference to the accompanying drawings in which:

FIG. 1 is a schematic section of part of a ducted fan gas turbine enginecomprising a prior art bearing arrangement;

FIG. 2 is an enlarged view on A in FIG. 1, but now showing a bearingarrangement in accordance with the present invention;

FIG. 3 is a further enlarged view on A in FIG. 1, but now showing abearing arrangement in accordance with the present invention;

FIG. 4 is an enlarged view on A in FIG. 1, but now showing analternative bearing arrangement in accordance with the presentinvention;

FIG. 5 is an enlarged view on A in FIG. 1, but now showing analternative bearing arrangement in accordance with the presentinvention.

With reference to FIG. 1, a ducted fan gas turbine engine is generallyindicated at 10 and has a principal and rotational axis 11. The engine10 comprises, in axial flow series, an air intake 12, a propulsive fan13, an intermediate pressure compressor 14, a high-pressure compressor15, combustion equipment 16, a high-pressure turbine 17, an intermediatepressure turbine 18, a low-pressure turbine 19 and a core engine exhaustnozzle 20. A nacelle 21 generally surrounds the engine 10 and definesthe intake 12, a bypass duct 22 and an exhaust nozzle 23.

The gas turbine engine 10 works in the conventional manner so that airentering the intake 11 is accelerated by the fan 13 to produce two airflows: a first air B flow which passes through the bypass duct 22 toprovide propulsive thrust and a second air flow into the intermediatepressure compressor 14. The intermediate pressure compressor 14compresses the air flow directed into it before delivering that air tothe high pressure compressor 15 where further compression takes place.

The compressed air exhausted from the high-pressure compressor 15 isdirected into the combustion equipment 16 where it is mixed with fueland the mixture combusted. The resultant hot combustion products thenexpand through, and thereby drive the high, intermediate andlow-pressure turbines 17, 18, 19 before being exhausted through thenozzle 20 to provide additional propulsive thrust. The high,intermediate and low-pressure turbines 17, 18, 19 respectively drive thehigh and intermediate pressure compressors 15, 14 and the fan 13 bysuitable interconnecting shafts 24, 25, 26.

The fan 13 is circumferentially surrounded by a fan casing 27, which issupported by an annular array of outlet guide vanes 28. Core engineinlet guide vanes 29 support a static structure 30, which is sometimesreferred to as a front bearing housing. An LP radial bearing 31 passesradial forces between the static structure 30 and the low pressure shaft26.

The core engine is surrounded by an intercase 32, which supports astatic intercase bearing housing 33. The intercase 32 also connects tothe outlet guide vanes 28. A LP thrust bearing 34 carries both axial(thrust) and radial loads between the LP shaft 26 and IP shaft 25. Anintermediate pressure (IP) thrust bearing 35 is supported by the staticintercase bearing housing 33 and supports the IP shaft in the vicinityof the LP thrust bearing 34. The IP thrust bearing 35 carries axial(thrust) and radial loadings. The static intercase bearing housing 33further supports an HP thrust bearing 36 which also supports the IPshaft 24. Other bearings are present within the gas turbine engine aswell known in the art, but have been omitted here for clarity.

Forwardly of the static structure 30 is a diaphragm 37 which defines achamber 38 between it and the static structure 30. The diaphragm 37comprises a rotating part 37R and a static part 37S that define a seal39 located between the two parts.

During engine operation high pressure air pressurises the chamber 38 andthe resultant pressure causes the diaphragm 37 and static structure 30to perform as a piston 40. This piston action causes a forward force 40Fon the LP shaft 26. The fan 13 also produces a forward force 13F. The LPturbine 19 produces a rearward force 19F.

In this prior art bearing arrangement, bearing end load management is bythe use of a rotating, air-pressurised piston 40, which balances therearward force 19F to a level that the location (thrust) bearings 34 canaccommodate. In a large civil aero engine, the LP bearing end load isthe load 19F that results from the large rearwards pull from the LPturbine 19 against the lesser forwards pull of the fan system 13F. TheLP system balance piston 40 is a means of applying further forwards load40F to bring the resultant load to within the capacity of a level thatthe LP location bearing 34 can manage.

In pursuit of improved engine performance, the core engine is reduced insize (diameter) and operated at increasingly higher overall pressureratio (OPR) to the maximum temperature limits of material technology.This has the effect of raising the pressure drop across the LP turbine19 which results in an ever increasing rearwards axial load 19F on theLP shaft 26. Thus, the pursuit of improved engine performance leads thedesigner to provide increasingly higher pressurised air behind the LPbalance piston.

The over-riding problem with this rotating air-pressurised piston 40 isthe performance penalty associated with air leakage at the rotating tostatic seal 39 interface. As engine performance increases and the LPturbine load increases further increases in the pressure of the piston40 are required, but inevitably increase the leakage through the seal 39and therefore the engine suffers a performance loss and an increase infuel consumption.

It is physically possible to include larger location bearings 34 in theinternal gearbox (effectively the bearings 34, 35, 36) in order toreduce the amount of load required of the LP system balance piston 40;however, this is of limited benefit as the internal gearbox bearingsoperate at relatively high speeds and the centrifugal contribution ofthe ball on bearing life soon becomes the dominant design driver.

Another possible solution is to swap over the bearings 31 and 34.However, not only does this affect the “fan blade off” and “windmill”characteristics of the engine but these relatively large bearings aresubstantially heavier themselves and mean that the surrounding supportstructures also require strengthening thereby increasing weight stillfurther. The vanes 28 and 29 will also require additional thickening tocarry the increased load which in turn causes aerodynamicinefficiencies.

The use of a very large bearing in the static structure 30 also becomescomplicated by the requirement to fail the bearing locating fuse duringa “fan blade off” event followed by the requirement to maintain axialretention of the whole LP system during the “windmill” fly home process.These two requirements can add significant amounts of extra weight tothis potential option.

Referring now to the present invention which is described with referenceto FIGS. 2 and 3, where the same components as the prior art have beengiven the same reference numbers. The gas turbine engine 10 comprises alow pressure system including the turbine 19 and the fan 13 (orcompressor) mounted on the first shaft 26. A bearing arrangementsupports the shaft 26 and is subject to a total end load. The bearingarrangement (34, 50) comprises a fixed bearing 34, a load share bearing52 and a bearing end load management system 50. The bearing end loadmanagement system 50 is capable of applying a variable load to the loadshare bearing to manage the total end load distributed between thebearings 34, 52. This bearing end load management system 50 does notsignificantly change the total bearing end load, but alters theproportion of the total bearing end load applied to each bearing. Notethat this is a very different arrangement to the prior art ‘piston’arrangement where the piston transfers significant thrust to the supportstructure so that the net thrust carried by the bearing simply reduces.

The bearing end load management system 50 comprises a static diaphragm54 that defines part of a chamber 56 and that is coupled to the loadshare bearing 52. Increased pressurisation of the chamber 56 loads thediaphragm 54 which in turn increases the proportion of the total endload on the load share bearing 52.

A second chamber 84 is formed forwardly of the diaphragm 54 andrearwardly of the fan's disc 86 which is connected to the LP shaft 26. Arotating seal member 88R is attached to the disc 86 to form a seal 88with a static part 88S that is attached to the inlet guide vane 29 inthe region of the fixed support 82.

Pressurised gas 58 is fed into the chamber 56 from a higher pressuresystem in this example the IP compressor 14, but could also be from theHP compressor 15. A conduit 60 is shown on FIG. 2 which leads from IPcompressor 14 and through the inlet guide vane 29. It is preferable tohave a number of conduits 60 around the circumference of the engine.Each conduit 60 is in continuous communication with the chamber 56 suchthat the bearing system 50 is effectively passive. When the engineincreases in speed the LP turbine 19 increases the total end load on theLP shaft. At the same time the compressor 14 increases its pressureratio and intrinsically delivers increasingly pressurised gas to thechamber 56. This increases the proportion of bearing end load onto theload share bearing 52. The fixed bearing 34 therefore does not become‘overloaded’ and is designed lighter and/or has an increased in-servicelife.

The chamber 56 is further defined by the static structure 30. The staticstructure 30 and the diaphragm 54 define between them two slidable seals65, 66 and which allow axial movement therebetween. The amount of axialmovement is relatively small perhaps being up to 10 mm but more usuallyabout 4 or 5 mm. The seal 65 is a ring seal and surrounds a piston 63.The piston 63 connects between the diaphragm 54 and the load sharebearing 52 as will be described in more detail later. A ‘squeeze film’contact 64 is formed between the load share bearing 52 and the staticstructure 30 both to allow for axial movement during load sharing whenthere is a change in pressure in the chamber 56 and also to providevibration damping to the LP system in the unlikely occurrence of a ‘fanblade off’ event.

One advantage of the present invention is that these slidable seals 65,66 are non-rotating and therefore may be designed and made with verytight gaps. Typically, these non-rotating seal clearances may be anorder of magnitude less than rotating seal clearances. Therefore thereis a very substantial reduction in the loss of high pressure gas withinthe chamber 56 compared to the prior art, leading to significantlygreater engine efficiency.

As can be seen in FIG. 3 the load share bearing, indicated generally as52, comprises a static race 68 connected to the diaphragm 54 and arotating race 70 connected to the LP shaft 26. The static race 68 isconnected to the diaphragm 54 via a piston 63, which is one of aplurality around the circumference of the engine. Spherical elements 72roll within the races 68, 70 and have a cage 74 to space them apart.Because this load share bearing 52 rotates between the LP shaft and astatic frame, centrifugal forces are far less problematic than the fixedbearing 34 which rotates relatively quickly between the LP and IP shafts26, 25. Therefore the load share bearing 52 is capable of carrying moreload than the fixed bearing 34. For an equivalent in-service life to thefixed bearing 34, the load share bearing will carry about 75% of thetotal end load of the shaft. Dependent on engine operating levels theload share bearing will almost always carry at least 50% and usuallybetween 55 and 85% of the total end load. It should be appreciate thatin the unlikely event of a failure of one of the bearings or otherassociated engine structure all of the end loading can be carried byeither bearing for at least the duration of the remainder of an aircraftflight cycle.

To improve the performance of the load share bearing a pipe 76, carryinggas flow 78, extends across the chamber 56 to allow substantially equalpressure either side of the chamber 56. Therefore the diaphragm andstatic structure will move relative to one another by consistent andknown amounts.

As can be seen most clearly in FIG. 2, the diaphragm 54 is an annulardisc and comprises two out-of-plane bends 80. These bends 80 are usefulto enable the slidable seal 66 to move only axially rather than tosubtend a radial displacement. As high pressure gas enters the chamber56 the force rotates the diaphragm 54 about its radially outer fixedsupport 82. The diaphragm 54 bends about the support 82 and begins toslide along the radially inward seal 66. The pressure in the chamber 56also ‘flattens’ the diaphragm slightly meaning that the seal 66 parts donot lose contact with one another. Furthermore, the corrugation(s) orbends 80 stiffen the diaphragm relative to its section thickness meaningit can be lighter weight than a flat section diaphragm of the samestiffness. In other words the corrugated shape is stiffer to axialmovement, at its radially inward positioned seals 65, 66, than a flatdiaphragm having the same sectional thickness. Thus the number and angleof the bends can be optimised for any given application to minimiseweight, linearity of movement of the seals and stiffness, which with thepressure in the chamber, ultimately dictates the load share betweenbearings 52, 34.

The flexibility/stiffness of the diaphragm 54 can be designed for eachapplication to suit the required load sharing proportions betweenbearings 34 and 52. FIG. 4 shows an alternative bearing arrangementcomprising a relatively stiff diaphragm 54. The diaphragm 54 comprises afurther bend 80 than that shown in FIGS. 2 and 3 and therefore has agreater second moment or area and hence stiffness in the axialdirection. The diaphragm 54 also comprises minimal straight or flatregions near to its supports. The change in pressure ΔP in the chamber56 imparts a force on the diaphragm 54 and therefore reaction forces onits radially outward support R_(static), the static structure andradially inward ‘support’ the rotor or shaft R_(rotor). With arelatively stiff diaphragm 54 the reaction forces are about equal and arelatively low load is applied to the shaft 26.

In FIG. 5 the diaphragm 54 comprises a flat portion 90 and a stiffportion 92 comprising a number of bends 80. The bends 80 give the stiffportion 92 its stiffness as described above. The flat portion 90,radially outward of the stiff portion 92, means that the diaphragm 54flexes more easily about the radially outward support. Thus in this caseR_(rotor) is significantly greater than R_(static) and a relatively highload is applied to the shaft 26.

Although the above described bearing system is passive, the pipe 60 mayinclude a valve 62 to fine tune the rate and extent to which the chamber56 is pressurised. Preferably, an array of pipes and valve are situatedaround the circumference of the engine. The valve 62 may be linked to anelectronic engine controller (commonly termed an EEC) and the extent towhich it is opened and closed is dependent on an engine function, forexample, the throttle position, a shaft speed or a load sensor on anyone of the bearings 34, 52.

The present invention may be further extended to a more active system ofbearing end load management by having a number of the pipes in the arrayof pipes attached to different parts of the or more than one compressor.This provides the capability of selecting a suitable gas pressure forthe desired bearing end load management between fixed and load sharebearing. In particular, bearing end load management may be made moreeffective during transient engine conditions, for example duringdeceleration.

It should be appreciated that the above described exemplary embodimentof the present invention can be applied to other shafts 24, 25 and thatthere may be more than one load share bearing system on any one machine.The present invention may be applied to any gas or steam turbine engineas well as other turbomachinery.

The present invention also lends itself to a method of operating abearing arrangement as described above. The method comprises the step ofpressuring the chamber above a predetermined limit such as a load on oneof the bearings 34, 52, an engine thrust setting, a shaft speed or otherengine condition.

1. A gas turbine engine comprising: a first turbine and a first compressor mounted on a first shaft and a bearing arrangement supporting the shaft about an axis, the bearing arrangement is subject to a total end load and comprises a fixed bearing, a load share bearing and a bearing end load management system that is capable of applying a variable load to the load share bearing to manage total end load between the bearings, the bearing arrangement is characterised by the bearing end load management system comprising a static diaphragm that defines part of a chamber and that is coupled to the load share bearing whereby increased/decreased pressurisation of the chamber loads the diaphragm which in turn increases/decreases a proportion of the total end load on the load share bearing.
 2. A gas turbine engine as claimed in claim 1 wherein the gas turbine engine comprises a second turbine and a second compressor mounted on a second shaft, the chamber is pressurised by gas from the second compressor.
 3. A gas turbine engine as claimed in claim 1 wherein the fixed bearing is mounted between the first shaft and the second shaft.
 4. A gas turbine engine as claimed in claim 1 wherein the load share bearing connects between the diaphragm and the shaft.
 5. A gas turbine engine as claimed in claim 1 wherein the chamber is further defined by a static structure, the static structure and the diaphragm are slidably sealed to one another to allow axial movement therebetween.
 6. A gas turbine engine as claimed in claim 5 wherein two seals are defined between the diaphragm and the static structure.
 7. A gas turbine engine as claimed in claim 1 wherein a pipe extends across the chamber to allow substantially equal pressure either side of the chamber.
 8. A gas turbine engine as claimed in claim 1 wherein the proportion of the total end load on the load share bearing is greater than 50%.
 9. A gas turbine engine as claimed in claim 1 wherein the proportion of the total end load on the load share bearing is between 55% and 85%.
 10. A gas turbine engine as claimed in claim 1 wherein the diaphragm is annular and comprises at least one bend.
 11. A gas turbine engine as claimed in claim 1 wherein the diaphragm is annular and comprises at least one flat portion.
 12. A gas turbine engine as claimed in claim 1 wherein a pipe comprises a valve to control the flow of pressurised gas into the chamber.
 13. A gas turbine engine as claimed in claim 1 wherein a number of pipes, each comprising a valve to control the flow of pressurised gas into the chamber, are connected to a number of differently pressurised gas sources.
 14. A method of operating a bearing arrangement as claimed in claim 1 wherein the method comprises the step of pressuring the chamber above a predetermined limit.
 15. A bearing arrangement of a shaft having an axis, the bearing arrangement is subject to a total end load and comprises: a fixed bearing, a load share bearing and a bearing end load management system that is capable of applying a variable load to the load share bearing to manage total end load between the bearings, the bearing arrangement is characterised by the bearing end load management system comprises a static diaphragm that defines part of a chamber and is coupled to the load share bearing whereby increased/decreased pressurisation of the chamber loads the diaphragm which in turn increases/decreases a proportion of the total end load on the load share bearing. 